The response of plants to microbial attack involves de novo synthesis of an array of proteins designed to restrict the growth of the pathogen. These proteins include hydroxyproline-rich glycoproteins, proteinase inhibitors, enzymes for the synthesis of phytoalexins, enzymes contributing to the reinforcement of cell walls, and certain hydrolytic enzymes such as chitinase and glucanase.
Plant defenses can also be activated by elicitors derived from microbial cell walls and culture fluids. In dicotyledonous plants, extensive studies have shown that microbial attack or elicitor treatment induces the transcription of a battery of genes encoding proteins involved in these defense responses, as part of a massive switch in the overall pattern of gene expression. The functional properties of the promoters of several of these dicotyledonous defense genes have been characterized. In contrast, relatively little is known about the inducible defenses in monocotyledonous plants, including the major cereal crops. For example, the transcriptional regulation of defense genes from monocotyledonous plants has not been examined.
Chitinase (EC 3.2.1.14) catalyzes the hydrolysis of the .beta.-1,4 linkages of the N-acetyl-D-glucosamine polymer chitin. Chitin does not occur in higher plants, but is present in the cell walls of many fungi. Chitinase, which exhibits complex developmental and hormonal regulation, has been found in many species of higher plants. In addition, chitinase activity is markedly increased by wounding, ethylene, or microbial elicitors. Furthermore, chitinase is involved in the hypersensitive resistance response to microbial attack. Purified plant chitinase attacks and partially digests isolated cell walls of potentially pathogenic fungi. It is this latter enzyme activity, rather than chitin-binding lectin activity, that is responsible for the inhibition of fungal growth. Chitinase and .beta.-glucanase exhibit synergistic antifungal activity in vitro. A number of pathogenesis-related proteins (also referred to as "PR proteins") have been found to be chitinases or glucanases.
Chitinase genes from a number of dicotyledonous plants (including bean, cucumber, potato, and tobacco) have been isolated and characterized.
Plant chitinases can be divided into at least three classes, based on amino acid sequence and cellular localization. Class I chitinases are basic isoforms which are structurally homologous and are primarily localized in the central vacuole. Basic chitinases contain a catalytic domain, and a cysteine-rich domain similar to rubber hevein. The hevein domain is thought to serve as an oligosaccharide-binding site. There is a variable spacer region between the hevein and the catalytic domains.
Class II chitinases are usually found in the extracellular fluid of leaves and in the culture medium of cell suspensions, suggesting that they are localized in the apoplastic compartment, consistent with a major function in defense. This hypothesis is supported by recent observations that some PR proteins are acidic chitinases.
Class III chitinases, such as a recently described cucumber chitinase, show no homology with either Class I or Class II chitinases, but are homologous to a lysozyme/chitinase from Parthenocissus quinquifolia. Class III chitinases are located in the extracellular compartment.
While chitinases from dicotyledons have been well characterized, and many of the corresponding genes have been isolated, there is little information available on the structure and expression of chitinase genes from monocotyledons.